Build in the Quality of the Supply

The power quality phenomena categories in IEEE Standard 1159 2009, “Recommended Practice for Monitoring Electric Power Quality,” are often used to define what to look for, how to look for it, and how to protect a piece of equipment from it. The following categories are excerpted from Table 2 of the standard:

1. Transients—impulsive and oscillatory
2. Short-duration rms variations—sag, swell, interruption less than 1 minute
3. Long-duration rms variations—interruption, sustained over-/undervoltage
4. Voltage imbalance (unbalance)
5. Waveform distortion—harmonics, interharmonics, notching, noise
6. Voltage fluctuations (also referred to as light flicker)
7. Power frequency variations

Depending on the type of loads in a facility and neighboring buildings, the percentage of electrical power consumed by susceptible loads (often incorrectly referred to as sensitive electronic loads) can be 30–50 percent or more. Add the complexity of networked communications between equipment within and outside the facility, and it may be prudent to attempt to improve the electrical supply quality to the facility’s equipment, based on each of the aforementioned categories.

• Strive to develop an equipotential ground plane in the facility by the sufficient grounding conductors and proper bonding with grounding—-grounding electrodes, equipment-grounding conductors, enclosures, raceways, conduit and building steel, in accordance with Article 250 of the National Electrical Code. A buried external ground ring can benefit other commercial and industrial sites as well.
• Install lightning protection on electrical and telecommunications circuits entering the facility. This is more than just putting in surge suppressors and lightning arrestors; the grounding conductors must be of sufficient gauge to handle the 10–40 kiloamperes typical of lightning strikes. The 100 kilohertz or higher impulsive transient requires a low-impedance path at those frequencies. Sharp bends—or worse, loops of extra wire—greatly increase the impedance at these frequencies.
• Install listed TVSS devices on critical circuits within the facility.
• Contact local utility for assistance with power factor (PF) cap switching oscillatory transients, unless facility owns PF caps.

Short-duration rms variations
• Install adequate conductor size with low-impedance connections to reduce voltage drop for the expected inrush current of the loads.
• Run separate circuits for critical loads versus large inrush loads, such as large horsepower motors. As a further step, install separately derived source circuits with an isolation transformer.

Long-duration rms variations
• To address utility-sourced long-duration events, ensure the backup generator is adequately sized, especially for the startup load current levels.
• Run separate circuits for critical loads requiring backup power.

Voltage imbalance
• If the imbalance is utility-side, change a tap on the service transformer.
• If the imbalance is facility-side, balance the current loading on each phase.

Waveform distortion
• Install adequate gauge neutral (grounded conductor), up to 200 percent of the phase conductor size.
• Use separate circuit/panelboards for significant harmonic current sources, including neutral (grounded) and grounding conductors.
• Consider installation of harmonic filters or zigzag, K-rated and/or electrostatically shielded transformers.
• For adjustable speed drives, consider use of higher pulse converters (12 or 24 versus 6), which typically have much lower harmonic current levels.
• Install adequate size grounding conductors, and follow the rules to minimize ground impedance at higher frequencies.
• In high electrical noise environments, use noise filters, and run conductors in grounded metal conduit with separate grounding conductors inside.

Voltage fluctuations
• If internal source, separate circuits for polluting equipment and filter.
• If external source, contact utility.

Power frequency variations
• This is rarely a problem if operating off of grid power.
• Verify the generator rating versus anticipated step load change.

The Copper Development Association states that, “The average additional cost to install an enhanced electrical distribution system, designed to the currently recommended practice, versus a ‘standard’ system has been estimated at 1 to 2 percent of the cost of construction. In retrofits, the costs of the solutions will run much higher, so the time to ‘do it right’ is in the initial design.”

With power quality related losses measured from tens of thousands to millions of dollars per hour, it wouldn’t take long for this small investment to pay off and reduce future losses for years to come.

BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.

About the Author

Richard P. Bingham

Power Quality Columnist
Richard P. Bingham, a contributing editor for power quality, can be reached at 732.287.3680.

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